The present invention relates to an optical fiber cable that is designed to minimize the level of temperature-dependent variations in signal propagation time. More particularly, the present invention relates to an optical fiber cable chiefly intended to be employed as signal transmission channels for high-speed data networks and phased array antennas.
In order to ensure complete synchronization in high-speed data networks or successful superposition of signals supplied to phased, array antennas, it is required that the propagation times of transmitted signals experience minimum levels of variation. If a change in temperature caused a variation in the propagation time of signals being transmitted through cables, the signals transmitted over respective routes are randomly offset to cause failure in synchronization. Furthermore, when such signals are superposed, the error due to signal offsetting will be increased.
Coaxial cables have been commonly used as signal transmission channels in high-speed data networks and phased array antennas but they have the problem of limited transmission capacity and transmission distance. Coaxial cables have the additional disadvantage that they are subject to temperature variations which are by no means small and will cause fluctuations in signal propagation time. The temperature coefficient of signal propagation time .tau. with the change in the temperature of the cable is given by the following equation: ##EQU1## The first term on the right side of the equation represents the effect caused by the increase or decrease in the physical length L of the cable, and the second term refers to the temperature-dependent change in the dielectric constant .sqroot..epsilon. of the plastic insulator in the coaxial cable. Each of these factors has a positive coefficient of the order of 10.sup.-5 (1/.degree.C.), so the overall effect of the increase in the temperature of the cable is delayed propagation of signals. This positive temperature coefficient of propagation time is a factor that limits the distance over which signals can be transmitted by the cable when it is used as a signal-carrying channel in a high-speed data network or phased array antenna.
With a view to avoiding the above-described problems associated with coaxial cables, the use of optical fiber cables as signal transmission channels has been proposed.
Replacement of coaxial cables by optical fiber cables is effective in avoiding the problems of low transmission capacity and high transmission loss. However, even optical fiber cables lack complete freedom from the problem of temperature dependency of signal propagation time. The temperature coefficient of propagation time .tau. for an optical fiber cable is given by the following equation: ##EQU2## As is clear from equation (2), the temperature coefficient of propagation time .tau. is subject to variations that are partly due to the linear expansion coefficient of the cable, the physical length L of which varies with temperature (this factor is represented by the first term on the right side of the equation), and partly to the temperature coefficient of the refractive index n of the optical fiber glass (this factor is represented by the second term). With optical cables in current use, the linear expansion coefficient ##EQU3## in the first term is in the range of from 1 to 2.times.10.sup.-5 (1/.degree.C.) and the temperature coefficient of refractive index ##EQU4## in the second term is 6.5.times.10.sup.-6 (1/.degree.C.) if the optical fiber is made of silica glass. Therefore, the temperature coefficient of propagation time .tau. is calculated to be ##EQU5## In other words, even an optical fiber cable employing low-loss and wide-band optical fibers is not capable of transmitting signals over a substantially longer distance than coaxial cables if it is used as a signal transmission channel in high-speed data networks or phased array antennas.